Commercial screening machines generally have a removable top cover which extends over and closes the top of the screen box in which the screen assembly is mounted. The removable top cover may be completely detachable from the screen box, or hinged to it along an edge. The particulate material to be screened is fed through the cover by an inlet chute which discharges it onto the top screen of the screen assembly. The cover encloses the particulate material being screened, preventing it from being shaken off the screen, minimizing dust, and preventing the entry of dirt and extraneous material. A gasket is often used between the cover and box to provide a better seal. From time to time it is necessary to open the cover, for instance to change or replace a screen or the entire screen assembly. Because the screen box is shaken with substantial force in operation, the cover is typically clamped to the box for movement with it.
Various forms of cover hold-down clamps have been proposed specifically for use on screening machines, including manually operated over-center hold-down clamps, for example of the type shown in Nolte U.S. Pat. No. 3,433,357. In the use of such clamps, a clamping member, adjustable by a screw, is manually engaged with the edge of the cover or frame and an arm is pulled from one side of a center position to the other side, so as to draw together the two members to be clamped. Such manual clamps provide a strong but inflexible clamping force. However, the adjusting screws of such clamps over time can become clogged with dust from the material being screened so that the screws cannot be easily turned to adjust the clamps. An additional problem that may be exhibited by screw type clamps is the tendency of the screw threads to "gall" or seize with extended use, also making it harder to turn the screws and adjust the clamps. With a screw type clamp, the clamping force applied increases as the screw is turned (i.e., with each degree of rotation of the screw). As the clamping force increases, the normal force and therefore the frictional force between thread surfaces also increases, making it harder to turn the screws and adjust the clamps. This problem can be exacerbated when the clamps, after being set up, are subjected to substantial heat in use, as for example when a hot material is being screened. Thermal expansion of the top cover-screen box assembly resulting from such heating subjects the threads to increased force, making it still more difficult to open the clamp. In some circumstances the force of thermal expansion on a tightly set up clamp can even warp or deform the clamp itself.
Spring type clamps have also been used to hold top covers on screen boxes. In such clamps, the clamping force is applied by a compression spring. Even though each applies clamping force differently, screw and spring type clamps can exhibit similar problems when used with a screening machine. For example, relaxation or setting of a gasket can lead to a number of problems shared by both screw and spring clamps.
Several screw or spring clamps are usually needed to hold down a top cover. In order to provide an approximately uniform clamping force at the several clamps around the periphery of the cover (so that the cover is not held too tightly at one area and too loosely at another), each clamp must be manually set or adjusted to provide roughly the same mechanical clamping force. In a large screening machine there may be a dozen or more clamps around the screen box. When several such clamps must all be set and adjusted for uniform force, it often happens that the force of the clamps first set changes by reason of subsequent gasket compression (i.e., flattening) as the other clamps are set so that it is thereafter necessary to go back and readjust the clamps which were first set. Such individual and repetitive adjustment requires substantial time, being done largely by trial and error.
Another problem arises during operation of the screening machine. With time, the gasket clamped between the cover and box can lose its resiliency (i.e., take a set) and therefore become thinner than its as clamped thickness. In fact, it can eventually wear away. Thus, as the gasket becomes thinner with continued operation of the machine, the clamping surface effectively moves away from the clamp. Once set, the clamping force exerted by a screw clamp or spring clamp decreases (as will be shown later, significantly) as the clamping surface moves away (i.e., as the gasket flattens and/or wears away). This problem can also be exacerbated when hot materials are screened. The heat generated during such processing can soften the gasket and speed up the reduction in gasket thickness.
Spring clamps can exhibit an additional problem as a result of hot material screening. The clamping force of a spring tends to drop, if it is exposed to high temperatures.